Wireless technologies have been developed to replace manual connections between devices with radio frequency (RF) signals, infra-red signals, ultrasonic signals and near field signals. Through the use of wireless technologies, portable computers can easily "connect" into a network simply by being placed in proximity to a device that supports the wireless communication that is already part of the network.
Each type of wireless technology has its own set of characteristics. For example, ultrasound networks tend to have very low data rates. In contrast, radio frequency (RF) networks provide relatively high data rates (10s of Megabits per second) over relatively long distances (hundreds of feet). RF networks have the disadvantage that RF technology tends to be the most expensive wireless technology. In addition, RF networks are subject to government regulations which vary substantially from one country to the next.
Infrared (IR) connections typically fall into one of two categories. The first category of IR connection is a low cost, short range (a few feet), line-of-sight connection between two IR capable devices. The second category of IR connection is a higher cost, longer range (30-40 feet), diffuse, omnidirectional connection between IR capable devices. Infra-red systems have advantages over RF systems in that data transmitted over IR signals is relatively secure, and IR is generally cheaper than wireless radio links.
Using IR technology, a point-to-point connection between two devices may be constructed for very low cost, with one or two emitter LEDs. If more range is desired, infrared emitters may be added to increase signal strength, at the expense of increased power requirements and cost. Diffuse systems, which have the largest expanse of range, may require up to 10 emitters to fully cover a room.
Numerous modulation methods have been developed for transmitting data using infrared signals. Modulation methods that are currently in commercial use include baseband pulsing, frequency shift keying (FSK), amplitude shift keying (ASK), phase shift keying (PSK), pulse position modulation (PPM) and burst-PPM. Each of these modulation methods involves tradeoffs between cost, signal distance, signal rate and ambient immunity. Ambient immunity, as the term is used herein, is the ability to receive information sent over infrared signals while rejecting ambient sources of light. Ambient sources of light include, for example, sunlight, fluorescent lighting and incandescent lighting
Baseband pulsing is typically less complex, and therefore less expensive than the other popular methods, but provides less ambient immunity and distance. Receivers for baseband pulsing can be as simple as a photodiode amplifier and a comparator.
ASK, FSK and PSK systems, on the other hand, typically cost more than baseband pulsing systems. The receivers of these types of systems generally require hardware that is more complex than the hardware required by baseband pulsing systems. Specifically, ASK typically requires a tank circuit and peak detector. FSK typically requires a quadrature frequency discriminator. PSK typically requires PLL based receivers. While ASK, FSK and PSK systems are more expensive than baseband pulsing systems, these systems also tend to have superior ambient immunity.
PPM is used in longer distance diffuse systems, where it is desired to project the maximum light output at the expense of a more complicated transmitter and receiver. For the reception of rectangular pulses, PPM may use a phase locked gated integrator to maximize the signal to noise ratio at the receiver. PPM also has inherent signal to noise advantages over constant carrier schemes. Like baseband systems, PPM systems show poor ambient rejection at low data rates. The ambient rejection improves as the data rate increases to a rate much higher than the ambient. This result occurs because filters may be used to block low frequency interference with relatively minor amplitude and phase distortion of the received signal.
In the design of a particular infrared device, cost, ambient immunity, distance, and data rate are all factors used to determine the appropriate protocol. Once a protocol is selected, the device is designed with an emitter and/or receiver that supports the selected protocol. In general, infrared devices that support one protocol cannot communicate with infrared devices that support different protocols. Therefore, most infrared devices can only communicate with a relatively small subset of other infrared devices.
Based on the foregoing, it is clearly desirable to provide a method and apparatus that allows an infrared device to communicate with a wider selection of infrared devices. It is further desirable to provide an infrared device that is capable of decoding signals from infrared devices that support different protocols.